Hydrothermal mineralization and alteration at the Globe Hill deposit, Cripple Creek District, Colorado
Date
1985
Authors
Trippel, Alan D., author
Thompson, Tommy B., advisor
Caddy, Stan W., committee member
Vaughan, John, committee member
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Abstract
The Globe Hill deposit is hosted by a porphyritic, subvolcanic intrusive which is composed of pyroxene-bearing alkali trachyte. It is emplaced within the Oligocene-age Cripple Creek diatreme-intrusive complex, which may be related to the Rio Grande rift system centered 75 km to the west. Four structural events occurred at Globe Hill. The earliest event (stage I) created a zone of upward-flaring hydrothermal breccia bodies which were later cut by a series of tectonic structures (stage II). A dike- or column-like body of intrusive breccia (stage III) subsequently invaded a major stage II shear zone, and a large, separate hydrothermal breccia body (stage IV) formed within the stage I zone. Separate hydrothermal fluids passed through each of the structural systems, and formed epithermal, low-grade, polymetallic mineralization along stage I, II, and IV structures; stage III structures remained unmineralized. Mineralization occurs as veins, breccia-matrix fillings, and disseminations through breccia fragments and adjacent wall rock. Stage I structures were mineralized by an assemblage composed dominantly of chalcedony-celestite-pyrite and trace oxides, telluride, phosphates, and base-metal sulfides. Early stage II veins are composed dominantly of quartzcelestite-pyrite and trace adularia, oxides, telluride, and base-metal sulfides. Wallrock adjacent to later stage II gangue-free structures contains halos of disseminated pyrite and trace oxides, telluride, and base-metal sulfides. The central core zone of the stage IV breccia is cemented by an assemblage of anhydrite-carbonate-celestite-fluorite along with trace pyrite, oxides, and base-metal sulfides; its peripheral halo is partially cemented by montmorillonite and minor amounts of fluorite, quartz, and hematite. Five separate alteration events are recognized. Each of the first four is hypogene and correspond to one of the major structural stages. The fifth event is related to supergene weathering. Alteration associated with the stage I hydrothermal breccias grades from a sericite-dominated zone within the breccia bodies to a chlorite-dominated zone in the surrounding wallrock. Wallrock adjacent to stage II tectonic structures has been altered to a sericite-dominated assemblage. Alteration of the stage III intrusive breccia primarily affected the matrix material and to a lesser extent the fragments, producing a chlorite-dominated assemblage. Stage IV central core zone breccia fragments were altered to a quartz-dominated assemblage which grades to a montmorillonite-dominated assemblage in the peripheral zone and wallrock. The last hypogene event as well as the supergene weathering have partially oxidized the deposit to a depth of over 270 m. The effects of supergene weathering are most pronounced along stage I and II structures where permeability is sufficient to allow for downward percolation of oxidizing surface waters. Mineralization and alteration assemblages associated with each structural stage have been used to infer parentfluid chemistry. Existing data suggest that the fluids were weakly to moderately alkaline, moderately oxidized, and of relatively low sulfur fugacity. Certain fluids associated with stage I, II, and IV structural stages must have contained appreciable amounts of Ca, Sr, phosphate, sulfate, and dissolved gases including CO2 and H2S. Zonal wallrock alteration assemblages indicate intermediate to weak metasomatic reactions adjacent to each structure. Boiling is interpreted as the major cause of mineral precipitation along stage I, early stage II, and stage IV structures. Evidence for boiling fluids includes the development of hydrothermal breccias, often along veins; explosion textures in vein quartz and celestite; the occurrence of adularia in some vein assemblages; and the highly variable vein fluid-inclusion homogenization temperatures and liquid to vapor ratios. The relatively restricted extent of most alteration zones suggests that groundwater mixing and metasomatic wallrock interactions occurred. These two processes could rapidly promote fluid-groundwater and fluid-wallrock equilibration, terminating further alteration. No evidence for fluid boiling was observed along the later stage II tectonic structures; in this case, metasomatic wallrock interactions were the major cause of mineral precipitation. The Globe Hill gold deposit may be the near-surface (hot-springs) expression of deeper, high-grade vein deposits like those located elsewhere in the district. Mineralizing fluids probably were convectively circulated by a thermal anomaly at depth beneath the Globe Hill area. The thermal anomaly and the presence of appreciable Ca and Sr in the fluids were probably derived from late alkali basalt igneous activity within the Cripple Creek diatreme-intrusive complex.
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Hydrothermal deposits -- Colorado -- Cripple Creek